1. Technical Field
This invention relates to a liquefaction process for a pressurized hydrocarbon stream using refrigeration cycles. More particularly, this invention relates to a liquefaction process for an inlet hydrocarbon gas stream using dual, independent refrigeration cycles having at least two different refrigerants.
2. Background of the Invention
Hydrocarbon gases, such as natural gas, are liquified to reduce their volume for easier transportation and storage. There are numerous prior art processes for gas liquefaction, most involving mechanical refrigeration or cooling cycles using one or more refrigerant gases.
U.S. Pat. Nos. 5,768,912 and 5,916,260 to Dubar disclose a process for producing a liquefied natural gas product where refrigeration duty is provided by a single nitrogen refrigerant stream. The refrigerant stream is divided into at least two separate streams which are cooled when expanded through separate turbo-expanders. The cooled, expanded nitrogen refrigerant cross-exchanged with a gas stream to produce liquified natural gas.
There is a need for simplified refrigeration cycles for the liquefaction of natural gas. Conventional liquefaction refrigeration cycles use refrigerants which undergo a change of phase during the refrigeration cycle which require specialized equipment for both liquid and gas refrigerant phases.
The invention disclosed herein meets these and other needs.
This invention is a cryogenic process for producing a liquified natural gas stream that includes the steps of cooling at least a portion of an inlet hydrocarbon gas feed stream by heat exchange contact with a first refrigeration cycle having a first expanded refrigerant and a second refrigeration cycle having a second expanded refrigerant that are operated in dual, independent refrigeration cycles. The first expanded refrigerant is selected from methane, ethane and other hydrocarbon gas, preferably treated inlet gas. The second expanded refrigerant is nitrogen. These dual, independent refrigerant cycles may be operated at the same time or operated independently.